The present invention relates to a control device for an internal combustion engine for detecting a knocking state produced in the engine and feedback-controlling the ignition timing, etc. to an optimum value in response to the knocking state.
It has been known that a continuation of a strong knocking state adversely affects the durability of the engine, etc., but a slight knocking state produced in a comparatively low RPM range improves output power and fuel consumption characteristics of the engine. Further, it has been known that the ignition timing is closely related to the occurrence of the knocking and in general, the more the ignition timing is advanced the more the knocking tends to occur.
Accordingly, a control device has been proposed in which in order to improve fuel consumption and output characteristics of the engine a knocking state produced in the engine is detected and the ignition timing is advanced and delayed in response to the detected knocking state so as to maintain a slight knocking condition.
Such a known device is based on the fact that when a knocking occurs in an engine, pressure changes are produced in the combustion chamber and pressure vibration components having a particular frequency are increased. In this known device a sensor for detecting the vibrations of engine body caused by the pressure changes in the combustion chamber is provided, from the detected signal of which sensor the particular frequency components are selected and the knocking state is detected from the magnitude of the selected particular frequency components.
In practice, however, the vibrations produced by the other factors than the knocking such as the movement of intake and exhaust valves and the rotation of a crank shaft include some vibration components having a frequency corresponding to said particular frequency, so that as the rotation speed of the engine increases the vibrations of the engine body and also the magnitude of the selected particular frequency components increase.
Therefore, in order to detect the knocking state correctly, the vibration components caused by the factors other than the knocking which increase with an increase in the engine rotation speed should be subtracted from the magnitude of the selected particular frequency components.
In the above mentioned prior art device, however, the subtraction of the components caused by the factors other than the knocking from the selected components is insufficient at a high rotation speed of the engine. Therefore, the prior art devices have a problem in that at a high rotation speed, the accuracy of the feedback control deteriorates and the ignition timing cannot be advanced properly.